Abstract

Implementing the capability to perform fast ignition experiments, as well as, radiography experiments on the National Ignition Facility (NIF) places stringent requirements on the control of each of the beam’s pointing, intra-beam phasing and overall wave-front quality. In this article experimental results are presented which were taken on an interferometric adaptive optics testbed that was designed and built to test the capabilities of such a system to control phasing, pointing and higher order beam aberrations. These measurements included quantification of the reduction in Strehl ratio incurred when using the MEMS device to correct for pointing errors in the system. The interferometric adaptive optics system achieved a Strehl ratio of 0.83 when correcting for a piston, tip/tilt error between two adjacent rectangular apertures, the geometry expected for the National ignition Facility. The interferometric adaptive optics system also achieved a Strehl ratio of 0.66 when used to correct for a phase plate aberration of similar magnitude as expected from simulations of the ARC beam line. All of these corrections included measuring both the upstream and downstream aberrations in the testbed and applying the sum of these two measurements in open-loop to the MEMS deformable mirror.

Pupil layout for the advanced radiography capability on the National Ignition Facility is displayed in Fig. 2a. The pupil represents four beam pairs with each of the beam pairs containing 1.99 kJ centered around 1.053 μm in a 5 ps pulse. Figure 2b shows the far-field pattern generated from this pupil assuming that all of the beams are pistoned correctly and have a perfect Strehl ratio.

Implementation of the interferometric adaptive optics system on the National Ignition Facility. The abbreviations stand for the following: BS, beam splitters; L, lenses; λ/2 and λ /4, half and quarter wave-plates, respectively.

Measurements examining the effects of applying tip/tilt to the MEMS device Fig. 3a represents the far-field intensity patterns at four separate values of tilt applied to the MEMS deformable mirror. Figure 5b displays the Strehl ratio as a function of tilt amplitude applied to the MEMS deformable mirror. The squares represent the Strehl ratios derived from the measured fa-field intensity patterns, the solid black line represents the theoretical Strehl ratio as a function of tilt and the solid grey line represents a vertical displacement of the solid black line.

Measurements examining the effects of piston errors between two adjacent rectangular apertures. The piston error was introduced by placing a microscope slide across the lower half of the MEMS device. The microscope slide introduced both a piston and tilt across one half of the MEMS device. Figure 6a represents the unwrapped phase measured with the microscope slide across the lower half of the MEMS. Figure 6b represents the far-field intensity pattern after the inverse phase was applied to the MEMS device.

Measurements examining the effects of Kolmogorov phase screen errors across the aperture. The wrapped and unwrapped phases are displayed in Fig. 7a and 7b, respectively. The far-field intensity patterns for the uncorrected and corrected far-field intensity measurements are shown in Fig. 7c and Fig. 7d, respectively.

Structure function for the unwrapped phase profile corresponding to the applied phase plate. The solid black line represents the measured structure function and the dashed grey line is the analytical Von Karman structure function with the Fried parameter equal to ro = D/18 and the outer scale length equal to L0=1.8D.

Background subtraction and residual charge data from the Sensors Unlimited wave-front sensor camera. Figure 9a represents the residual charge on the wave-front sensor camera as a function of camera reads. The laser was applied to the focal plane array before the second camera read. Figure 9b represents the residual charge on the wave-front sensor camera after the average intensity has been subtracted.